One might also call one of these a solitary bee house. I like this design better because with a stack of logs or lumber (from what I see) you also build a containment frame.
This one I can just set up on a large log, hang from a post, or post on a post. As you can see, I used a plain log 6″ or so in diameter, face flattened with a hatchet and smoothed with a plane. I stuck with 4, 6, and 8 16ths holes. For the roof, I set a circular saw to its max depth at a 45 degree angle, broke off the waste, finished the peak with a handsaw, and used some durable cardboard that a cable company left in the neighborhood, which I glued and nailed down. If necessary, the overhang can hold wire mesh to keep birds from pecking.